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“Simply put, unleaded fuel burns less smoothly than leaded and the toluene and benzene, which is added to the fuel to act as a lead substitute, burns to form what are called peroxides. Unfortunately peroxides are inherently unstable and explode, forming shock waves that cause detonation. This explains why engines designed to run on unleaded from the outset feature sophisticated knock sensors, which retard the ignition setting at the onset of harmful detonation.”

https://www.classiccars4sale.net/classic-car-how-to-guides/r...

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Right, but that article goes on to talk about various non-lead substitutes you should add... I know why unleaded vs leaded, but there are substitutes for lead that are designed for use in classic cars but don't actually contain lead, that's what I was asking the difference of.

Basically, why use actual lead in gas when lead substitute exists and is recommended for classic cars as an additive to unleaded fuel?

I believe that lead in petrol / gasoline still exists as there are not one but two effects from lead.

One is that lead helps promoting burning of the gasoline versus explosion or knocking.

Two is that lead is a great lubricant for the valve surfaces, preventing them eroding, leaking or sticking as they open / close. Eroding causes the engine to lose compression and efficiency, and sticking (as I have experienced first-hand) can cause a major engine failure by breaking the mechanism that opens/closes the valve.

This is correct, but it's also been overstated how important the lubricating properties of leaded gasoline were in these engines - while the harmful health effects were understated. [1]

[1] www.walshcarlines.com/pdf/mechanicalimplications.d4e.pdf

I've found it varies by engine type. International Harvester light line SV engines for example definitely benefit from lead additives to fuel. Zinc additives in oil is another 'be kind to your engine' additive https://zddplus.com

Engines of previous eras were designed to leverage the 'lead' in gasoline as a lubricant and act as a 'cushion' to the valve seats and valves as the fuel flowed through the engine. Not adding a 'lead' substitute can damage your valves, and ethanol is to be avoided at all costs to protect the engine.

Engine 'knock' (multiple explosions on a single piston ignite cycle as different pockets of fuel mixture ignite) is also minimized by having 'lead' in the fuel.

Have '48 Chevy - can confirm that lead-substitute exists and classic cars will run with it. I never use the car without it so its possible the stuff doesn't actually do anything.

I suspect(and this is just my guess) that if you have a true classic that is worth millions of pounds, you don't want to risk running it on additives - so for those extremely rare occasions when you want to start the engine(car shows usually) you might as well run it on the £5/litre genuine leaded petrol.

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